Polylactate (PLA) is a typical biodegradable polymer derived from lactate, and is highly applicable in general-purpose or medical polymers. In recent years, PLA has been synthesized by polymerizing lactates produced by microbial fermentation, but this direct polymerization of lactates can produce only PLA having a low molecular weight (1000 to 5000 daltons). In order to synthesize PLA of 100,000 daltons or more, low molecular weight PLAs obtained by the direct polymerization of lactates may be polymerized into higher molecular weight PLAs using a chain coupling agent. However, this method of preparing a higher molecular weight PLA has problems in that its processes are complicated due to addition of a solvent or a chain coupling agent, and removing the solvent or chain coupling agent is difficult as well. The method of producing high molecular weight PLA widely used in recent years includes converting lactate into lactide, and subjecting a ring of the lactide to a ring-opening condensation reaction to synthesize PLAs.
Meanwhile, polyhydroxyalkanoate (PHA) is polyester which is accumulated in a microorganism as an energy or carbon source storage material when there are excessive carbon sources but a lack of other nutrients such as phosphorus, nitrogen, magnesium and oxygen. The PHA is known as an alternative for conventional synthetic plastics since it has similar properties to conventional synthetic polymers derived from petroleum and shows perfect biodegradability.
In order to produce PHA from a microorganism, an enzyme converting metabolic products of the microorganism into a PHA monomer and a PHA synthase synthesizing a PHA polymer using the PHA monomer are essentially required.
The PHA synthase synthesizes PHA using hydroxyacyl-CoA as a substrate. However, there are no cases where PLA and a copolymer thereof are naturally produced or produced in recombinant cells or plants since hydroalkanoate, such as lactate hydroxylated at the carbon-2 position, is not suitable for substrate specificity of PHA synthase. In order to provide lactyl-CoA, the inventors can successfully synthesize PLA and a PLA copolymer using a Clostridium propionicum-derived propionyl-CoA transferase and a polyhydroxyalkanoate synthase of Pseudomonas sp. 6-19 using the lactyl-CoA as a substrate (Korean Patent Application No. 10-2005-0043798).
However, the conventional developed microorganisms still have low capability to produce PLA and a PLA copolymer, and thus development of a microorganism having improved capability to produce the PLA and PLA copolymer is required.